The present invention relates to methods and apparatus for refrigerating products and more particularly to improved, high performance tunnel refrigeration devices.
Tunnel refrigeration devices have been utilized for freezing or chilling products such as food or other materials. Typically, these devices are comprised of an elongated, insulated chamber having an open entrance and exit and an endless belt conveyor extending through the tunnel chamber. The conveyor belt is adapted to receive the product at the tunnel entrance and to pass the same through the tunnel while a refrigerant such as carbon dioxide or liquid nitrogen is introduced into the tunnel to refrigerate the product. The conveyor belt may be comprised of a wire or chain link structure or may take the form of a foraminous belt. The refrigerant is generally introduced into the upper portion of the tunnel and is mixed with the tunnel atmosphere by means of top mounted fans which are effective to also direct the tunnel atmosphere into contact with the product thereby cooling the same. A typical prior art, refrigeration device is illustrated in U.S. Pat. No. 4,086,783.
Another type of tunnel refrigeration device is one in which a plurality of belt runs or "passes" are utilized to translate a product being refrigerated through an enclosed chamber. One such multi-pass tunnel refrigeration apparatus is illustrated in U.S. Pat. No. 3,708,995 which utilizes side mounted fan means for the purpose of developing a helical flow of tunnel atmosphere in a direction substantially transverse and extending vertically across all conveyor surfaces supporting products being refrigerated in the tunnel. A refrigerant supply is introduced into the fans to thereby develop a chilled tunnel atmosphere and these devices do not utilize top mounted fans as described in the refrigeration tunnel hereinabove. In yet another refrigeration apparatus utilizing an endless conveyor belt, products are passed in a spiral pattern through a refrigeration chamber as illustrated in U.S. Pat. No. 3,733,848 which is assigned to the assignee of the present invention. In this spiral freezer, elongated centrifugal fans are mounted vertically in the refrigeration chamber for the purpose of blowing a refrigerated atmosphere over all levels of the spiral conveyor to thereby refrigerate products being passed through the chamber. Again, this device does not utilize top mounted fans nor the introduction of a refrigerant downwardly onto products being passed through the refrigeration chamber.
While the above-described refrigeration tunnels are generally effective to refrigerate products, the rate at which products can be refrigerated to a desired extent is limited and cannot be readily increased without extensive structural changes. For example, increasing conveyor belt speed will accelerate the rate of passing product through the tunnel device, but unless sufficient heat is removed from the tunnel in a shorter period of time, simply increasing belt speed will not assure that a product is refrigerated to a desired extent. In addition, although the rate of introduction of refrigerant into a tunnel may be increased, it is necessary to increase the capacity (i.e. speed and bladesize) of the top mounted fans to assure that the refrigerant and tunnel atmosphere are mixed to thereby sublime or vaporize the refrigerant as it is important to preclude the refrigerant, such as CO.sub.2 snow, from being carried out of the tunnel with the product which, in turn, results in more efficient utilization of the refrigerant. However, it has been found that attempts to increase fans capacity to accommodate a greater flow of refrigerant will increase the downward velocity of the tunnel atmosphere generated by the top mounted fans which in turn results in this atmosphere rebounding from the bottom of the tunnel upwardly through the conveyor belt and actually lifting the product off the conveyor belt as it is being passed through the tunnel. This effect of lifting product is particularly noticeable during the accelerated freezing of products such as hamburger patties and as a consequence, patties can be actually thrown from the belt and disfigured so that a product of unacceptable configuration and appearance is discharged from the tunnel freezer. Similarly, it has not been feasible to increase the throughput or capacity of tunnel refrigeration devices without extensive structural modifications such as increasing conveyor belt length or area. However, these modifications are expensive and significantly increased the capital cost of tunnel refrigeration devices. Thus, there is a clear need for improved tunnel refrigeration devices which enable a greater rate of refrigerating product to a desired extent yet do not require increased tunnel belt lengths or other major structural modifications.